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Astron. Astrophys. 351, 21-30 (1999)

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5. Results and discussion

5.1. Results of structure function analysis

Examination of 6 light curves of the NGC 1275 nucleus in the spectral band near [FORMULA] 5200 Å in 1982-1987 and in the UBVRI system in 1989-1994 allows us to draw the following conclusions:

  1. SFs for 2 different periods of the observations and different spectral regions do not show an ideal one-process form: no SFs have a lower plateau, the upper plateau is broken, and different slopes are presented for time lags of less and more than one day.

  2. The lowest limit of variability is not observed because a first plateau of SFs has not been revealed. Therefore we suggest that the minimum time-scale of the optical variability of the NGC 1275 nucleus could be less than 5 min (i.e., the time resolution of our observations). It would be obtained using a shorter time resolution and a higher level of the threshold of detection.

  3. Shapes of SFs obtained show that during 1982-1994 there were at least three processes in the NGC 1275 nucleus causing the optical variability: two processes with time scales of less than one day, and the third one with a time scale of at least 4 years.

  4. Two processes causing intranight variability are more pronounced than the process causing variability in a scale of years. They are characterized by a rather steep SF (0.25[FORMULA] b [FORMULA] 1.19): a mixed process of flicker-noise and shot-noise, or pure shot-noise is observed. The third process, operating on years time lags is pure flicker-noise, or absent: the slope "b" of SFs is in the range 0.06-0.14.

  5. The slopes of SFs obtained in the UBVRI system in 1989-1994 for the intranight variations are not equal to each other. The highest slope is obtained for the U and I bands: b = 1.0-1.2. This value exceeds the slope of the SF in the V band by (2-3)[FORMULA]. It can be interpreted as the existence of independent activity of the nucleus in the near infrared compared to the ultraviolet and visual regions of spectrum (on a level of (2-3)[FORMULA]). The same result was obtained by I.Pronik et al. (1998): an examination of the calculated SDs of the variable flux for each night showed that there were about 40% of nights with SD [FORMULA] 3[FORMULA] for the UBVR bands and 53% for the I band.

  6. Maxima and minima on SFs, corresponding to time lags more than one day exhibit the presence of separate flares on time lags of days, months and years. There is only one common minimum for all 6 SFs, it is located on time lags 0.9 days[FORMULA] dt [FORMULA]2.5 days. We suppose that there are possible periodical or quasi-periodical signals in the NGC 1275 nucleus within the interval (0.9-2.5) days. It is impossible to ascertain the more exact value of this quasi-period using SF analysis.

5.2. Evolution of a process in the nucleus of NGC 1275 causing its optical variations

According to the V light curve shown in Fig. 1, during 1968-1979 the NGC 1275 nucleus was in an enhanced state, rapid and powerful flares were observed to occur up to twice a year. In 1979 the optical source made a transition to a new and quiescent stage which has lasted up to the present date. After 1980 the decay of the nucleus brightness is continuing, but more slowly. Estimates showed that the average V brightness of the nucleus reduced to the diaphragm 5" in 1989-1994 was about 40% lower than it was in 1982-1987.

In Table 5 we show the parameters of the SFs of the UBV variable fluxes for the time interval 1968 - XII.1979, which we have calculated using data by Lyuty (1980) and Nesterov et al. (1995). One can see that the slopes of the SFs are 0.34[FORMULA] b [FORMULA] 0.40, the confidence levels of the correlations in all cases are equal to 1. The slopes show that in 1968-1980 in the NGC 1275 nucleus a mixed process of flicker-noise and shot-noise was operated.


[TABLE]

Table 5. Structure function parameters of years variability of the NGC 1275 nucleus in 1968-1980.
Notes:
Signs in Table 5 are the same, as in Tables 2-4.


A comparison of the data of Tables 4 and 5 permits us to suppose that the flux decrease of the NGC 1275 nucleus in the UBV bands from 1968-1980 to 1989-1994 is accompanied by a decrease of the slopes of the SFs from (0.3-0.4) to (0.12-0.14). The SF slopes showed that the processes have evolved from mixed flicker-noise and shot-noise in 1968-1980 to classical flicker-noise in 1982-1994. This evolution was in agreement with the decrease of the nucleus brightness.

Pronik et al. (1998) registered the evolution of microvariability of the NGC 1275 nucleus on time scales less than of one day, too. From 1982-1987 to 1989-1994 an increase of the number of nights with SD[FORMULA] 3[FORMULA] was observed from 8% to [FORMULA]40% , showing the increase of the duty cycle of the nucleus. Intranight activity of the nucleus is increased when its general flux decreases. These results need to be interpreted by theoretical models.

5.3. Comparison with the radio data

The relationship between the optical and radio fluxes of blazars was investigated by Hufnagel & Bregman (1992). The optical and radio data are positively correlated in the sense that the optical flux variations precede those in the radio region, implying a significant processing of the optical-emitting plasma before it becomes a radio-emitting plasma. There is a dissimilarity between the optical and radio variations, which indicates the possibility that the connection may not be linear and there is a difference in the structure of the emitting plasma in the two domains. Based upon the results that the optical flux variations precede those of the radio region, the authors conclude that the two regions are physically related although on substantially different length scales. The variability data indicate that the plasma emitting optical and ultraviolet radiation has a dimension about an order of magnitude smaller, than the size of the plasma emitting in the radio domain. One of the models of the nucleus flux variability is the inhomogeneous jet of blazars in which shocks propagate along the jet which produce individual radio outbursts and may hold promise to explain optical variations as well.

The slopes of the SF for the variable optical flux of the NGC 1275 nucleus in 1982-1994 are different for the intranight and years time scales. The most powerful nuclear process in the time interval 1982-1994 operated on time scales of less than one day. The slopes of the SF for the intranight variability were in the range 0.25[FORMULA]b[FORMULA]1.19, but for the larger time scales 0.06[FORMULA]b[FORMULA]0.14. Unfortunately we cannot compare the intranight and long-term radio variations. There is poor information on time scales of less than one day in the radio domain. But it is important that intranight and night-to-night variations of the radio emission have been registered. Joint observations at [FORMULA] 1.35 cm at the Crimean observatory and Metsahovi (Finland) in June 1978 (Efanov et al.,1980, 1981) showed a variability of about 23% over one day. Terasranta et al. (1987) measured in Metsahovi an intranight flux variability of the NGC 1275 nucleus (3C 84) in April, May 1984 at [FORMULA] 3.9 mm of about 4%, and night-to-night variations of up to 10%. But these data are not enough for the investigation of SF and for drawing a conclusion on the character of the intranight radio variability.

The structure functions of the radio source 3C 84 were calculated only for time scales of years. Hughes et al. (1992) have obtained SFs with time lags from 2 weeks to 10 years using sets of the observations in a time interval of more than 20 years from 1965, at frequencies of 4.8 GHz, 8 GHz and 14.5 GHz. Averaged for 3 frequencies, the value of the slope b is equal to 1.6. The maximum time of the correlated nonperiodical variability equals more than 10 years.

Lainela & Valtaoja(1993) using sets of observations at 22 GHz and 37 GHz, obtained at Metsahovi from 1980 to 1986, have calculated an SF with a slope b = 1.5, the time scale for 37 GHz was 3.75 years, and for 22 GHz it was more than 9.5 years.

The comparison of the characteristics of SF of the NGC 1275 nucleus in the radio and optical domains shows that on time scales of years nonperiodical variations in the radio are near to shot-noise: b [FORMULA] (1.5-1.6); in the optical during the maximum activity in 1968-1980 - the process is a mixed shot-noise and flicker-noise (0.3[FORMULA]b[FORMULA]0.4), but in a quiescent state in 1982-1994 the slope corresponds to pure flicker-noise (b [FORMULA]0.14). SFs in the optical are flatter than in the radio domain.

Results obtained for the nucleus of NGC 1275 cannot be distinguished from the results obtained for the other blazars: the flatness of the SF and a decrease of the time scale of variability from the radio to the optical were characteristic (Hufnagel & Bregman, 1992). SF analysis showed that the nature of the variability of optical and radio fluxes is different. It may be described by different power-law forms: shot-noise in the radio, and a mixed process of shot-noise and flicker-noise in the optical.

5.4. Quasi-periods

Basko & Lyuty (1977) looked for possible periods in the UBV variability of the NGC 1275 nucleus using 1966-1976 observations. They revealed six probable quasi-periods from 15 to 340 days on a low level of probability. Our data of SFs for the 1982-1994 observations do not support any of these quasi-periods. A quasi-period of 2.3 days was suspected by Merkulova & Pronik (1985) using spectral observations with the 6-m telescope on January 1977. It lies within the range of the common minimum in all SFs, obtained for 1982-1994 observations (Fig. 4), showing the probability that a quasi-period exists equal to [FORMULA] 2.2-2.3 days. This quasi-period may be supported by future investigation.

It is not excluded that the several-day quasiperiodic modulation with time scales slightly larger than one day is a common property of the flux variations of blazars. For instance, the V light curve of one of the best observed blazars OJ 287 revealed quasi-periods of 8.5 days and 22 days according to Turner et al. (1994), and of 12.6 days - according to Lehto (1994). Gonzalez-Perez et al.(1996) gave a review of possible interpretations of such events: "spotty disk", binary singularity, et al. An examination of the nature of quasi-periods is very important for the definition of the nucleus model. It is necessary to continue this analysis for the NGC 1275 nucleus using new observations.

5.5. Implications of the observational data for various models

Many studies have suggested that noise-type variations of AGNs are caused by instability either in disk or in jets, connected with a black hole in the sources. From Table 5, the SF slopes of year-scale variability of the nucleus of NGC 1275 during the active phase in 1968-1980 are in the range (0.34-0.40), near to 0.35 evaluated by Kawaguchi & Mineshige (1998) for the optical flux variability of the quasar 0957+561. They concluded that the disk instability model is favored for these quasar variations, because fluctuations on days scales are hard to produce by other models. Below we discuss the data obtained on the characteristics of the NGC 1275 nucleus variations from the point of view of current models.

Accretion onto a black hole (ABH). Some authors concluded that the optical continuum of the NGC 1275 nucleus has a nonthermal nature (Oke, 1968; Anderson, 1970; Wampler, 1971; Babadjaniants et al., 1972; Shields & Oke, 1975; de Bruyn & Sargent, 1978). A power law spectrum of the optical continuum [FORMULA] [FORMULA] [FORMULA] and polarization up to 5% constitute the observational evidence for this conclusion. In particular, Edelson & Malkan (1986) modelled the optical continuum of the NGC 1275 nucleus by summing the nonthermal part with a power coefficient - 1.53 and the thermal part from the black hole having T = 20 000 K. In this model the optical thermal emission equals only 12% of the nonthermal one. They made the supposition that the luminosity of both synchrotron and thermal emission arises from the accretion at the Eddington limit of the luminosity on central black hole. Based on the accreting nature of variability of blazars, Elliot & Shapiro (1974) forecast a time scale of the NGC 1275 nucleus's microvariability of the order of 5 min. Edelson & Malkan (1986) calculated a lower limit of the Schwarzschild radius for this source equal to 2[FORMULA] 10 12 cm. Region 3 [FORMULA] Rg [FORMULA] 10 for the NGC 1275 nucleus is equal to (6-20)[FORMULA] 1012 cm, corresponding to (3-10) light minutes. One can expect a flux variability on such a time scale. It does not contradict our data on intranight variability of the optical continuum flux of the nucleus.

Instabilities in shock-in-jet (SJ) model is also possible the explanation of the intranight variations of the NGC 1275 nucleus. Heidt & Wagner (1996) obtained parameters of SFs for 34 radio selected BL Lacertae objects in the optical R band; the slopes of the SFs are within the range 0[FORMULA] b [FORMULA] 2.5, with a mean value of 0.8 and a dispersion of 0.6. The typical time-scale lies in the range between 0.5 and 5 days. The duty cycle of these objects is at least 0.8. They concluded that intraday variability is a characteristic property of BL Lac objects. Our data on microvariability of the optical flux of the NGC 1275 nucleus gave 0.25 [FORMULA] b [FORMULA] 1.19 and do not contradict these data. In general, the results were explained by the standard model where shocks are propagating down a relativistic jet: the shock-in-jet model. There will be a high degree of microvariability, if jets are bent and turbulent (Marscher,& Travis 1991; Wagner, 1991). The explanation of the variability is based on the scenario of shocks propagating within the jet entering a turbulent region.

But in both models (ABH and SJ) of the NGC 1275 nucleus's microvariability there exist unclear points. It is interesting that the power processes acting on the short-term and long-term variability of the optical continuum of the NGC 1275 nucleus correspond to the general brightness of the nucleus in different ways. Long-term variability operated dependent on the brightness of the nucleus: from 1968-1980 to 1994 the brightness of the nucleus decreased and the power of the long-term variability decreased, too, from mixed shot-noise and flicker-noise to pure flicker noise. On the contrary, the high level microvariability in 1989-1994 increased compared to that 1982-1987, while the general brightness of the nucleus was not enhanced. According to our data (Pronik et al. 1998) when the flux varies by 20-30%, the amplitude of microvariability changes by a factor of 4. Is it possible that the process increases when the brightness of the nucleus declines in terms of accretion onto a black hole and shock-in-jet models? It is a question for future investigations.

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Online publication: November 2, 1999
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